There are light measuring apparatus known wherein the direction and the extent of detuning of the measuring bridge is indicated not by an indicating instrument but by the evaluation of the relative brightness of two signal lamps. The measuring bridge is indicated to be balanced when both signal lamps burn with equal brightness.
A drawback of these apparatus is that the evaluation of the relative brightness in the proximity of the balanced state of the measuring bridge is very difficult and also with small brightness differences, not always detected by the naked eye in the same measurement. The measuring bridge may already be extremely detuned to such an extent that the values ascertained are outside the allowable error tolerance.
For the elimination of this drawback, an arrangement has been proposed wherein three signal lamps are employed, of which one serves for the positive indication of the balanced condition, while the other two light up depending on the detuning of the measuring bridge.
This arrangement indeed eliminates the uncertainty, conditioned by the evaluation of the lamp brightness during the balancing procedure, but requires a certain expenditure for structural components, which make the light measuring apparatus more expensive and makes its utilization more complicated in that the illuminating effect of three lamps must be observed.
The object of the invention is to provide a light-measuring apparatus, without instruments, of the initially mentioned type wherein the brightness differences (evaluated only with difficulty) of both signal lamps do not influence the balancing of the measuring bridge above the allowable error tolerances; and this object should be realized with the fewest possible components to facilitate extensive miniaturization of the apparatus.